Filtration mask

ABSTRACT

A FACE MASK HAVING HIGH AND PROLONGED FILTERING EFFICIENCY SPECIALLY ADAPTED FOR SURGICAL USED FORMED FROM A FLTERING WEB COMPOSED ENTIRELY OF SYNTHETIC ORGANIC FIBERS INCLUDING FILTERING FIBERS 0.5 TO 6 MICRONS IN DIAMETER AND A NON-FUZZY FACE CONTACTING LAYER FORMED FROM A POROUS SMOOTH-SURFACED THERMOPLASTIC FILM.

Allg. 6, 1,14 D. J, MAYHEW Re. 28,102

FILTRATION MASK Original Filed Feb. 24, 1970 I NVE NTOR.

gam-,WJ Mar/few United States Patent Oce Re. 28,102 Reissue-d Aug. 6,1974 28,102 FILTRATION MASK Delbert J. Mayhew, White Bear Lake, Minn.,assigner to Minnesota Mining and Manufacturing Company, St. Paul, Minn.

Original No. 3,613,678, dated Oct. 19, 1971, Ser. No. 14,210, Feb. 24,1970, which is a continuation-impart of application Ser. No. 623,369,Mar. 15, 1967. Application for reissue June 21, 1973, Ser. No. 372,152

Int. Cl. A62b 23/06 U.S. Cl. 12S-146.2 10 Claims Matter enclosed inheavy brackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

ABSTRACT F THE DISCLOSURE A face mask having high and prolongedfiltering efficiency specially adapted for surgical use formed from afiltering web composed entirely of synthetic organic fibers includingfiltering fibers 0.5 to 6 microns in diameter and a non-fuzzy facecontacting layer formed from a porous smooth-surfaced thermoplasticfilm.

This case is a continuation-impart of my copending application Ser. No.623,369, filed Mar. 15, 1967 now abandoned.

This invention relates to face masks having high and prolonged filteringefiiciency, particularly suitable for use as surgical masks.

The surgical face masks which have been standard for decades and whichare in wide usage even today are composed of layers of gauze coveringthe nose and mouth. Even though the bacteria filtration efficiency ofsuch masks is quite low initially and becomes even lower after a veryshort wear period, these masks have been widely used because they areeasy to breathe through. Substitute material such as foamed rubbers, ornonwoven webs formed from conventional synthetic organic fibers, havesuffered from these same disadvantages. Substitute materialsincorporating small diameter inorganic fibers, such as asbestos orglass, while providing good breathing and filtering characteristics,tend to present health hazards due to the possibility of inhalation ofsmall inorganic particles. Furthermore, even very fine glass fibers,e.g. 3 or 4 microns in diameter, can cause skin irritation. The use ofvery ne organic fibers has not been found practical prior to the presentinvention because webs made therefrom tended to be difficulty handleableand uncomfortable due to fuzziness of the material; or if made morecoherent by heat and pressure tended to present too high a resistance tobreathing.

The present invention provides face masks having high and prolongedfiltering efficiency. The masks of this invention are sufficientlyeconomical to be disposable after use. These masks combine highfiltering efficiency with wearing and breathing comfort, avoidance offuzziness or other skin irritating properties, and the avoidance of thehazard of inhaling irritating particles. The masks of this inventioncombine a filtering layer formed from tiny synthetic organic fiberswhich provide high small-particle filtering efficiency and yet haveenough void volume to provide ease of breathability or low resistance toair flow in combination with a smooth very porous fuzz-free,tearresistant, moisture-resistant surface layer for contacting the face.The inner, face-contacting surface layer is nonabsorbent, and thusallows moisture and other small particles to pass therethrough withoutbecoming wet to the touch, even after prolonged wear. Masks of thisinvention are free of objectionable odors and in finished form arepreferably substantially free of solvents or odor causing adhesives. Dueto the nature of the preferred synthetic organic polymeric materials,the masks of this invention are resistant to deterioration from themoisture normally present in exhaled air. Preferably the outer side ofthe mask is also covered by a porous fabric. These layers are preferablyheld together only around the edges, but could also be fastened togetherat other points if desired. The mask is provided with tie strings,elastic, or the like, to hold the filtering portion of the mask over theusers nose and mouth and is preferably provided with a strip of softdeformable metal or the like along the edge of the mask designed tocover the users nose, to provide a close fit which minimizes leakage ofexhaled air around the edge of the mask.

The invention will be further explained with reference to theaccompanying drawings wherein:

FIG. 1 is a perspective view of a mask of the present invention;

FIG. 2 is a cross-sectional view of the mask of FIG. l taken along line2-2; and

Fl-G. 3 is a perspective view showing a mask of this invention in use.

Face mask 10 consists of a central, usually rectangular body portion 12intended to cover the lower portion of the face of the user. The mask isfurther provided with ties 14 at the ends thereof for securing the maskover the face of the user. A pleat 16 may be formed in the body portionof the mask to permit unfolding or widening thereof to cover generallythe nose, mouth, and chin of the user. In the folded condition the maskis more compact for packaging, shipment and storage. Edge binding 18 maybe conveniently used to hold the layers forming the body portion 12 ofthe mask together, as well as holding in place the ends of the pleat 16.Ties 14 may also be extensions of the edge binding 18, or may beseparate fabric or elastic strips if desired. Stitching 20 is apreferred means of holding the edge binding and layers of the mask bodyportion in place. Heat sealing, adhesives, or tapes and the like couldalso be used.

As seen in FIG. 2, body portion 12 of the mask is preferably formed ofthree layers. The central layer is filtering web 22, a web of tinythermoplastic organic fibers. A surface of filtering web 22 issuperimposed upon a surface of face contacting layer 24. Face contactinglayer 24 is a thin soft flexible self-sustaining nonabsorbent highlyporous discontinuous film of fused and coalesced nonwoven, inert,thermoplastic, synthetic polymeric fibers. The face contacting surfaceof layer 24, i.e. the exposed surface, is the surface opposite to theinner surface which contacts filtering web 22. This exposed surfacefeels smooth and is skinlike in nature. The outer layer 26 of the maskmay be formed from any porous fabric, woven or nonwoven. In analternative form of this invention, the outer layer 26 is omitted. Outerlayer 26 is less essential than face contacting layer 24, sincefuzziness and even loss of filtering fibers from the resulting exposedside of filtering web 22 will not seriously annoy or inconvenience theuser. However, it is preferred to include outer layer 26.

Filtering web 22 is formed from long, very fine synthetic organic fibershaving an average diameter of about 0.5 to 6 microns, the very bestcombination of filtering efficiency and breathability being obtainablewith fibers havingaverage diameters of l to 3 microns. Preferably thesefibers` are in the form of loosely held-together bundles or ropes,because in this form of the fibers are much more resistant to mattingtogether or compaction into a dense paperlike form which would causeexcessive resistance to air ow through the web. Alternatively, theindividual tiny fibers can be mixed with larger polymeric fibers whichserve to prevent undue compaction.

The tiny fibers are preferably formed in accordance with the proceduredescribed in Naval Research Laboratory Report No. 111,437, dated Apr.15, 1954, entitled Manufacture of Superfine Organic Fibers. Thisprocedure involves extruding a ne stream of molten polymeric materialinto a stream of heated air or other fluid which causes a breakdown orattenuation of the extruded material into tiny fibers. Fibers of thistype are known in the art as sprayed or blown fibers. They are alwaysstaple rather than continuous, because they are broken into lengths onthe order of about 0.5 to about cm. Preferred polymers for forming suchsprayed fibers for face masks include polypropylene, polyethylene,polyesters, polyamides such as nylon, polycarbonates, polyphenyleneoxide, and fluorinated polymers such as trifluorochloroethylene. Otherrionabsorberit, slippery feeling hydrophobic thermoplastic polymers mayalso be used` The fiber di` ameter rather than the composition appearsimportant in providing filtering characteristics so long as the materialis relatively moisture-insensitive, odor-free, and dimensionally stableunder ambient conditions. The fibers should have a minimum length ofabout 0.5 cm. to avoid loosening or breaking off of short fragile liberfragments which could cause irritation to the wearer if inhaled. Fibersof this length also assist in forming a web which is sufiicentlycoherent to be handled. The fibers are generally found to be raveled andintertwined sufficiently to provide a web which is sufficientlydimensionally stable. [t has been found that loosely raveled bundles orropes of fiber can be obtained by collecting the bers at a distance of20 to 40 inches (about 5() to 100 cm.) from the nozzle orifice. Thesebundles of ropes provide a web with good cohesiveness and filteringcharacteristics. Alternatively, a closer collection point can be usedwhereby the fibers tend to mat as individual fibers rather than bundlesor ropes. These mats may tend to be somewhat more compacted than themats comprising bundles or ropes, but nevertheless are suitable for usein this invention.

It is somewhat surprising that a 100 percent fine fiber (fibers lessthan 6 microns and preferably less than 3 microns in diameter) web is socohesive and so porous, particularly in view of the fact that the web ispreferably collected and combined with the other elements of thisinvention substantially as is. That is, the filtering web is notsubjected to special internal bonding procedures, eg. hot pressing.Furthermore, it would be expected that the fine fibers might becomeunduly compacted, even in the absence of heat or pressure. Apparentlythe sprayed fibers are tacky enough to spontaneously bond tenaciously toone another without further heating or other treatment. This tackinessand self-bonding causes the formation of a web of extremely fine staplefiber which, even when the fibers are individual rather than intertwinedinto bundles or ropes, does not require the use of coarser reinforcingfibers to insure either cohesveness or porosity.

In order to provide the needed ease of breathability, the filtering webshould have a void volume of at least about 87 percent. The ease ofbreathability can be correlated with the pressure drop which occurs whenair is forced through the filtering portion of the web at a controlledrate. As a standard test for checking the suitability of webs for use inpracticing the invention, a disc ofthe filtering material 31/16 inch indiameter is used, and air is pumped through the material at a flow rateof 1 cubic ft. per minute. The difference in air pressure between oneside of the filtered disc and the other is measured with a manometer ina conventional manner. The pressure drop (measured in inches of water)should be in the range of about 0.15 to 0.85 inches H2O. Filters causinga pressure drop under these conditions of less than 0.15 are found tohave poor filtering efficiency, while those causing a pressure drop ofover about 0.85 are unacceptable because of difficulty in breathingthrough a mask formed therefrom. Optimum results were obtained usingwebs which caused a pressure drop of 0.15 to (L5 inches H2O.

The characteristics of the mask are also affected by the thickness ordensity of the filtering material. lt is a feature of this inventionthat a very low density mask can be made wihout sacrificing any otherneeded properties such as filtering efficiency. lt is preferred forcomfort and breathability that the ream weight (the weight of 320 squareyards of material) of the filtering material be substantially less than55 pounds. Fibers such as polypropylene are very low in density, and al0 or l5 lb./320 sq. yd. filtering material can be made, though withdifficulty, from such fibers. Ream weights less than 3() lb./320 sq.yd., e.g. 20-25 lb./320 sq. yd. are readily achieved in practice and arepractical from a manufacturing standpoint.

The inner face-contacting layer can be constructed by forming a nonwovenlayer of randomly oriented thermoplastic fibers, and subjecting thematting to a heat treatment (e.g. about 300 F. (149 C.) in the case ofpolypropylene) sufficient to soften the fibers and cause them to adhereto one another to form a self-sustaining web. This web is then furthertreated to form a smooth surface free of fuzz or other irregularities bypressing it against a smooth heated surface such as that of a heateddrum or roller. Backup pressure rollers or similar means can be used toinsure that the matting is uniformly pressed against the surface of thedrum. The temperature of the heated smooth surface should be sufficientto cause melting of the surface fibers of the matting, and sufiicient owto form a smooth flat surface without losing their fibrous form. Thefused fibers are then coalesced in this smooth coridition. In the caseof polypropylene, a temperature of about 330o F. (166 C.) is suitable.Other smooth heated surfaces, such as traveling belts or the like, canbe used instead of the heated drum. This treatment of the mattingproduces a surface having a smooth fuzz-free feel, while yet maintainingthe porous discontinuous nature of the film. The film should have anaverage of about 10 to 40 holes per square millimeter of surface, andthe holes preferably have an average area of [0.1] 0.01 squaremillimeter or less, and constitute 10 to 40 percent of the total area ofthe surface, and preferably 20 to 30 percent thereof. The large numberof tiny irregularly shaped holes permits easy flow of air through thelayer and thus provides for easy breathability while forming acontaining envelope for the filtering web which prevents inhalation ofloose fibers or irritation due to skin contact with the filtering layer.The thermoplastic fibers from which the layer is formed should be nogreater than about 4 denier and preferably are 3 denier fibers, and theresultant facecontacting layer has a weight of 3.1 to 12.9 grains per 24square inches (00013-00054 grams per square centimeter), and preferablyfrom 4 to 6 grains per 24 square inches (0.0016 to 0.0025 grams persquare centimeter). The absence of adhesives or binder resins in the webavoids odors normally found in webs bonded by the use of such means.

The invention will be further explained with reference to the followingexample in which all parts are given by weight, unless otherwiseindicated.

EXAMPLE A filtering web made up of bundles of microfine polypropylenefibers was formed. A polypropylene resin having a melting point of 333F. (168 C.) was extruded through an orifice having a diameter of 0.020inch. The extruder was operated at a temperature of 700 F. (371 C.), theextrusion die temperature being 560 F. (293 C.). The extruder wasoperated at a rate of 14 lbs. (6.35 kg.) of resin per hour. The resinemerging from the die was immediately blasted with hot air at 800 F.(427 C.), which was discharged from a '3A-inch (1.9 cm.) opening at apressure of 5 p.s.i. (0.34 atmospheres). The web was collected at a rateof 2l ft. per minute (6.4 meters per minute) at a distance of about 38inches from the extrusion die. The fibrous web thus collected consistedof loosely raveled bundles or ropes, of tiny individual fibers havingdiameters of 0.5 to 3 microns, and an average diameter of about 1.5microns.

A face contacting web was also formed from polypropylene. Three denierpolypropylene stable fibers were formed into a nonwoven matting, usingconventional apparatus. The matting had a weight of 5 to 7 grains per 24square inches (0.0021 to 0.003 grams per square meter). The matting waspassed over a smooth surfaced heated drum having a surface temperatureabout 330 F. (166 C.) using a pressure roller to compact the matting toinsure uniform heat transfer to the surface of the matting. The fibersnearest the surface of the drum melted, but retained their fibrous form.On coalescing, a smooth non-fuuy surface having a human-skinlikeappearance with a myriad of tiny pores or holes therethrough was formed.

An outside cover web for the mask was formed from rayon fibers. 1.5denier viscose rayon fibers were processed on a garnet machine whichformed a fluffy nonwoven web having fibers more or less aligned in thedirection the web traveled. The following binding solution was appliedby means of a compactng wool roller to bind the rayon web into arelatively tear-resistant form:

Parts Polyvinyl alcohol (Shawanigan Gelvatol 20-60) 50 Water 600 Bluepigment 0.44 Green pigment 0.26 Urea-formaldehyde resin Al2(SO,)3 2

The urea-formaldehyde resin and the Al2(SO4)3 serve to insolubilize thepolyvinyl alcohol resin, providing a fuzz-free surface, and also bindthe pigments to the web, preventing color bleeding. Upon drying, the webhad no perceptible odor.

Rectangular pieces of the three webs were assembled with the filteringweb sandwiched between the facing webs. Strips of the rayon cover webwere folded over to form narrow strips about 1.6 cm. in width. Thesestrips were then folded around the edges of the assembled webs andstitched into place as shown in the drawings. The edge binding wasextended about l5 inches (38 cm.) beyond each side of the top and bottomedges of the mask body to provide tie strings. A 3A inch (0.48 cm.) widedouble layer of aluminum foil backed pressure sensitive adhesive tapeabout 6 inches (15 cm.) long was enclosed in the binding along the majorportion of one edge to form a deformable nose band which will retain theshape of the users face contour to minimize air leakage.

The masks, when worn with the polypropylene facing web against thewearers face, were comfortable, provided ease of breathability, and didnot interfere with normal conversation. The mask was tested forefficiency on a capped masked human subject, in accordance with theGreen-Vesley in vivo method described in the Journal of Bacteriology,Vol. 83, No. 3, March 1962, pp. 663-667. The subjects head was placed inan enclosed chamber and any expelled moisture droplets collected onagar-agar. The mask had an efliciency of 97.7 percent as defined by thisin vivo test method.

What is claimed is:

1. In a face mask having a body portion adapted to cover the nose andmouth and means for securing said body portion over the nose and mouth,the improvement wherein said body portion comprises:

a face contacting web comprising a nonwoven, porous self-sustaining,flexible, fibrous matting having an exposed smooth, fuzz-free surface,said exposed surface having an average of about l0 to 40 holes persquare millimeter of surface, said holes having an average area of lessthan about [0.1] 0.01 square millimeter and constituting 10 to 40percent of the area of said surface;

a porous filtering web consisting essentially of intertwinedthermoplastic organic fibers, said fibers being uniformly distributedthrough said filtering web and having diameters ranging from 0.5 to 6microns;

said porous filtering web being superimposed upon the surface oppositesaid exposed smooth fuzz-free surface of said face contacting web.

2. A face mask according to claim 1 wherein said body portion of saidmask includes a retaining layer, said retaining layer comprising aporous fabric; said face contacting web, said porous filtering web, andsaid retaining layer being arranged such that said filtering web iscontained between said surface opposite said exposed smooth fuzz-f reesurface of said face contacting web and a surface of said retaininglayer.

3. A face mask according to claim 1 wherein said thermoplastic organicfibers are polypropylene fibers.

4. A face mask according to claim 1 wherein said thermoplastic organicfibers have an average diameter of l-3 microns and a length of at leastV2 centimeter.

5. A face mask according to claim 1 wherein the said exposed surface ofsaid face contacting web consists of thermoplastic fibers fused togetherat their points of contact with each other and coalesced while retainingtheir fibrous form, thus forming numerous irregularly shaped tinyopenings in said exposed surface to permit easy breathing therethrough.

6. A face mask according to claim 1 wherein: the void volume of saidfiltering web is at least about 87 percent, and the weight per unit areaof said filtering web is less than about pounds per 320 square yards.

7. A face mask according to claim 6 wherein said weight per unit area isless than about 30 pounds per 320 square yards.

8. A face mask according to claim 6 wherein the pressure drop caused bysaid filtering web ranges from about 0.15 to 0.85 inches of water whenair is pumped through a 3]/16th inch diameter disc of said filtering webat a fiow rate of l cubic foot per minute.

9. A face mask according to claim 8 wherein said pressure drop is about0.15 to about 0.5 inches of water.

l0. ln a face mask having a body portion adapted to cover the nose andmouth and means or securing said body portion over the nose and mouth,the improvement wherein said body portion comprises:

a face contacting web comprising a nonwoven porous,

self-sustaining, fiexible, fibrous matting having an exposed smooth,fuzz-free surface, said exposed surface having an average of at least 10holes per square millimeter of surface, said holes having an area ofless than [0.1] 0.01 square millimeter and constituting at least 10percent of the area of said surface;

a porous filtering web having a void volume of at least about 87 percentand a ream weight of less than 55 pounds per 320 square yards of areaand consisting essentially of intertwined thermoplastic organic fibers,said fibers being uniformly distributed through said filtering web andhaving diameters ranging from 0.5 to 6 microns; said porous filteringweb being superimposed upon the surface opposite said exposed smoothfuzz-free surface of said face contacting web.

(References on following page) patent.

References Cited The following references, cited by the Examiner, are ofrecnrd in the patented le of this patent or the original UNITED STATESPATENTS 8 3,285,245 11/1966 Eldredge et al. 128-156 3,316,904 5/1967Wall et al. 12S-146.6

FOREIGN PATENTS 871,661 6/1961 Great Britain.

CHARLES F. ROSENBAUM, Primary Examiner U.S. Cl. X.R. 55-512

